Thanks for the suggestions...
I will make it clearer in my pages what the power considerations are for the regular UNO when using the external power jack.
Lets discuss the "Common Ground Point" issue more. There certainly are multiple levels of this when high-frequency signals are involved. Individual circuit boards will/should have their own ground plane. They in turn may plug into a backplane that has a ground plane. A large system may have a large backplane with multiple "ground" planes if the same backplane includes both high power boards and low-level analog circuits.
There may be large subassemblies (such as the device interface board for a chip tester) that have their own separated ground planes and very short signal leads. I have been the design lead on systems like that, with 2 rack-mounted computers plus a couple distributed microcontrollers and the XYZ control of the prober. Planning the "Ground Point" issue early-on is really important. It's easier when you are on a raised floor and you can run 3 inch wide copper strap :-)
Let's try to find a good explanation that will be most helpful to people here making typical one-Arduino 'systems' of some type. It those cases, assuming the mixture of Arduino and some high-power controlled devices, I think the negative/common point of the power supply/supplies is the best initial suggestion.
Maybe we should pick this up as a separate thread on Project Guidance? Hmmmmm.
What do you think about all this?
I bet a Mars Rover has this same problem.
But sooner or later there has to be a Common Ground Point.